Measurement of the Coulomb quadrupole amplitude in the γ*p Δ(1232) in the low momentum transfer region Nikos Sparveris Massachusetts Institute of Technology Hall A Proposal PR

u d uud u γ * Μ1, Ε2, C2 Μ 1+, Ε 1+, S 1+ π o p(qqq) I = J = 938 MeV Δ(qqq) I = J = 1232 MeV Ν Δ(1232) Spherical M1 Deformed M1, E2, C2 Deformation signal Experimental confirmation of the deviation of the proton structure from spherical symmetry is fundamental and has been the subject of intense experimental and theoretical interest Studied through the measurement of the electric and Coulomb quadrupole amplitudes (E2,C2) in the predominantly M1 (magnetic dipole-quark spin flip) N Δ(1232) transition The issue

Experimental activity: MAMI, Bates (low-Q 2 ), JLab (Hall A, B & C) mapping from Q 2 =0.06 (GeV/c) 2 up to 6 (GeV/c) 2 Theoretical activity: dynamical calculations, phenomenology, ChEFT, Lattice (Sato-Lee, DMT, MAID, SAID, Pascalutsa-Vanderhaeghen, Alexandrou et al) Quark model predictions are 30% too low for M1 and an order of magnitude lower for the quadrupole amplitudes This issue of the quark core and pion cloud contributions has been addressed in a meson exchange model by Sato & Lee – the model quantitatively makes up for the deficiencies of the quark model The dynamic Sato-Lee calculations are in excellent agreement with the CLAS data But: Sato-Lee not in agreement with the low Q 2 data taken at Bates and MAMI near the predicted peak of the pion cloud contribution at 0.1 (GeV/c) 2. No data available lower than Q 2 = 0.06 (GeV/c) 2 There are some discrepancies between the MAMI and the Bates data at Q 2 = (GeV/c) 2 that make the picture unclear at a crucial point. The status

effect of quark core + pion cloud Sato-Lee calculation effect of quark core

Resonant amplitudes in the low Q 2 region

Extracting the signal Separation of the partial cross sections with measurements at various azimouthal angles p o Η(e,e p)π o R LT R L +R T R TT R LT Multipole Truncation Multipole Truncation Model interpretation Model interpretation Multipole decomposition Multipole decomposition CMR, EMR Data Background Signal

Capability to place the spectrometers in small angles and high resolution spectrometers The lowest Q 2 measurements taken at MAMI (Q 2 =0.06 (GeV/c) 2 ) were constrained by space limitations (lower limits for the 2 MAMI spectrometers are 23 o and 15.1 o ) The 2 HRS spectrometers in Hall A can go down to 12.5 o thus providing access to lower Q 2 values. Experiment requirements: Hall A standard equipment only The 2 HRS spectrometers for e and p detection respectively (with their standard detector packages: VDCs, scintillators, Cherenkov, lead-glass) A 6 cm LH 2 target Beam: E ο =1115 MeV and I=75 μΑ (beam energy will stay constant during the experiment) Beam energy can be easily adjusted around the above value to accommodate beam energies of other experiments Why Hall A ?

Φ pq = 0 o Φ pq = 180 o σ LT = ( σ (Φ pq =180 o ) - σ (Φ pq =0 o ) ) / 2 v LT p o The Experiment H(e,e p)π o

Kinematical Settings 20% dead-time & 99% detection efficiency have been assumed 47 hrs production + 8 hrs calibrations + 17 hrs config. changes = 72 hrs 6 cm LH 2 target, E o = 1115 MeV, I=75 μA 8.5 hrs 9 hrs 29.5 hrs

Trues / Accidentals 2 ns timing window & 60 MeV Missing-mass cut around pion mass

Phase space (W,θ pq,Q 2 ) will be matched for Φ pq = 0 o, 180 o measurements analysis bin size: ΔW = ± 4 MeV, Δθ pq = ± 2.5 o, ΔQ 2 = ± 3 * – 4.5 * (GeV/c) 2 theoretical calculations folded over the acceptance for the extraction of point cross sections cross section uncertainties : statistical < ± 1%, systematic < ± 3%, avg.-to-point < ± 0.4% σ LT uncertainty < ± 8% (depending on kinematics) resonant amplitudes will be fitted to the cross sections CMR (statistical+systematic) uncertainty < ± 0.20% to < ± 0.28% (from Q 2 =0.125 to 0.04 (GeV/c) 2 ) contributions from background amplitudes from all available models will be introduced to the fits Model uncertainty introduced to the CMR < ± 0.30% in all cases σ LT will be extracted down to Q 2 = (GeV/c) 2 and unmatched cross sections in (W,θ pq,Q 2 ) will be extracted down to Q 2 = (GeV/c) 2 Data analysis

Phase space : Q 2 = (GeV/c) 2 ΔQ 2 = ± (GeV/c) 2 ΔW = ± 4 MeV Δθ pq = ± 2.5 o analysis bin widths ΔW cut = ± 5 MeV

Q 2 = 0.04 (GeV/c) 2 Q 2 = 0.09 (GeV/c) 2 ΔQ 2 = ± (GeV/c) 2 ΔW = ± 4 MeV Δθ pq = ± 2.5 o ΔQ 2 = ± (GeV/c) 2 ΔW = ± 4 MeV Δθ pq = ± 2.5 o Phase space ΔW cut = ± 5 MeV

Projected Results: Q 2 = (GeV/c) 2 Bates results seem to overestimate the MAMI ones at Q 2 =0.125 (GeV/c) 2 Disagreement in the description of the parallel cross section as a function of W

Q 2 = 0.06 (GeV/c) 2 Q 2 = 0.20 (GeV/c) 2 Mainz data Q 2 = (GeV/c) 2 proposed measurements W-dependence at low Q 2

Projected Results Q 2 = 0.04 (GeV/c) 2 Q 2 = 0.09 (GeV/c) 2

Projected Results: CMR

going even lower ? Q 2 = (GeV/c) 2 … working on it 900 MeV Q 2 = (GeV/c) 2 pion cloud ahead will take an extra 12 hrs (beam on target+config. changes) + E o = 900 MeV

CMR will be precisely mapped from Q 2 =0.125 (GeV/c) 2 down to 0.04 (GeV/c) 2 this experiment will provide the lowest Q 2 CMR measurements and the most precise ones in the low momentum transfer region cross sections will be also be extracted down to (GeV/c) 2 discrepancies of other labs (Bates/MAMI) will be resolved strong constrains to the most recent theoretical calculations will be provided valuable insight to the mechanisms that contribute to the nucleon deformation Summary Request: The 2 HRS spectrometers (with their standard detector packages) A 6 cm LH 2 target Beam: E ο =1115 MeV and I=75 μΑ (central E o value adjustable if needed) 3 days of running (including production, calibrations and configuration changes)

Measurement of the Coulomb quadrupole amplitude in the γ*p Δ(1232) in the low momentum transfer region

BACK – UP SLIDES

Results: Q 2 = 0.20 (GeV/c) 2

Results: Q 2 = (GeV/c) 2 Latest compilation of Bates data and comparison with Mainz data

Deformed The Nucleon is Deformed Deformed Spherical Deformed Spherical

Ποιοτική διερεύνηση MAID στα αποτελέσματα CMR = R CM (MAID) 1 CMR = R CM (MAID) 0.5 CMR = R CM (MAID) 0 ( … spherical ) R CM (MAID) ~ - 6.5%